Heat exchange devices, or heat exchangers, are devices for transferring heat from one medium to another, typically from one fluid to another or to the environment, without allowing the fluids to mix. Some examples are: automobile radiators; air conditioners, which use both a condenser and an evaporator; and steam and hot water radiators, which are used to produce heat. In order to prevent mixing of the fluids, or liquids, a barrier is provided between the two liquids or media. Many different heat exchanger barrier designs are used. In a “plate and frame” design, which is very compact, two liquid streams pass on opposing sides of one or more plates. The total heat transfer surface may be increased by increasing the area of plates and the number of plates. In a “tube and shell” design, one stream of liquid flow passes through the tube(s) and the other through the remaining space inside a shell that surrounds the tubes. A special subcategory the tube and shell design would be an immersion coil type design, such as a heating coil in a tank. However, both the “plate and frame” and “tube and shell” designs are susceptible to fouling and clogging. These drawbacks are considerable when considering applications relating to treatment of waste water.
A particular application of heat exchangers is in the area of waste water heat reclamation or “recovery”. There are many examples of both “tube and shell” and “plate and frame” waste water systems. However, many of these systems often require a filter, because they are susceptible to clogging and/or fouling due to the nature of their design. Also, in addition to the heat exchanger itself, it is often necessary to have an elaborate apparatus to perform the actual waste water treatment. Some of these systems include coils, but these coils are often a part of a tube and shell design, such as an immersion coil.
Helical coil-on-tube heat exchangers have been in use for some time. This type of heat exchanger typically consists of a single coil that is wrapped around a tube. Prior coil-on-tube heat exchangers have been used as direct-fired water heaters, in which combustion takes place within the tube, warming the liquid in the coil. Coil-on-tube heat exchangers are also used for waste-water heat recovery.
Typical liquid flow rates have traditionally been modest using the single coil design. More recent applications of this class of heat exchanger, such as wastewater heat recovery, have resulted in much higher liquid flow rates. Coil-on-tube type heat exchangers have a significant advantage in wastewater applications as the center tube allows the waste water to pass through easily without clogging. Production rates for single-coil-on-tube heat exchangers are low and provide good performance.
However, in many applications, desired flow rates result in a large pressure loss in single-coil designs. The loss is generally proportional to the distance travelled in the coil, the second order of the flow rate, and is inversely proportional to the cross-sectional area. When long lengths of coil are required, the resulting pressure loss is not acceptable for many applications.
By increasing the number of liquid pathways on the coil side of the heat exchanger, pressure loss can be reduced. Coil-on-tube heat exchangers having multiple coils exist, with different designs being typically used for different applications. The number of coils used depends on the maximum desired flow rate. The higher the desired flow rate, the more coils are needed to keep pressure losses to a reasonable amount. For example, in a single residential installation, such as most houses, a ½ inch nominal tube is used for a coil, and 1 to 2 coils are used. For apartment buildings, 2 to 4 coils are typically used, and in commercial settings (such as health clubs, etc.), several coils are typically used by manifolding heat exchangers. Each design is not necessarily limited to a given application (a 4 coil unit could be used for a commercial or a residential application). The important thing is that the number of coils be high enough to keep the pressure loss low enough for the flow rate in a given application.
FIG. 1 illustrates a conventional heat exchanger with multiple coils, each provided as single-coil helixes. In such a known design of a heat exchanger 10, a center tube 12 is provided having a center tube inlet end 14 and a center tube outlet end 16. In the two-coil heat exchanger 10 of FIG. 1, a first coil 18 is located around a first portion of the tube 12 and a second coil 20 is located around a second portion of the tube 12. The first coil 18 has a first coil inlet end 22 provided near the center tube outlet end 16 and a first coil outlet end 24 provided near the mid-point of the length of the center tube 12. The second coil 20 has an inlet end 26 provided near the mid-point of the length of the center tube 12 and an outlet end 28 provided near the center tube inlet end 14. The use of the terms inlet and outlet ends above presumes that a liquid flow in the center tube 12 is in a different direction that the liquid flow in the first and second coils 18 and 20.
The total liquid inflow for the coils is thus divided into two so that a portion of the incoming liquid flows to each of the two coils 18 and 20, entering at an the inlet end thereof. This reduces the overall liquid pressure loss through the coils as compared to the single coil design. However, to accomplish this, a header or manifold is required to connect the multiple coils together, since the inflow points and outflow points of the heat exchanger are spread out over the length of the center tube. The different coils will not perform their function without the header, since without the header the inflow could only reach the first coil, and the outflow of the first coil could not output at the outflow end of the center tube. The header can include an inflow header 30 and an outflow header 32, connecting the inflow and outflow ends of the coils, respectively.
Although the coils are able to treat flows of liquid in parallel with each other, the coils are themselves placed on succeeding distinct longitudinal sections of the center tube. As mentioned above, the treatment of parallel flows of liquid requires that the heat exchanger include the header. The need for a header requires additional production time, as well as additional installation time.
Some heat exchanger designs have been found to be more efficient than the multiple-coil coil-on-tube heat exchanger shown in FIG. 1. “Counter-flow” (or “contra-flow”) heat exchangers are known to be one of the most efficient, or effective, classes of heat exchangers. In a counter-flow heat exchanger with a plurality of coils, the temperature difference between the liquids is substantially constant along its length. Generally, a cold water flow enters a coil at one end of the heat exchanger, and a warm water flow enters another coil at the other end of the heat exchanger. The warm water flow provides heat to the cold water flow, and the warm water flow gets cooler as it travels along the heat exchanger, while the cold water flow gets warmer as it travels along the heat exchanger. If the cold and warm water flows were to enter the heat exchanger at the same end, there would be a large temperature difference at that end, and a much smaller temperature difference at the other end. This parallel flow case is limited to a maximum heat exchanger effectiveness of about 50%.
Therefore, taking a look at the multiple-coil-on-tube heat exchanger of FIG. 1, it is not a true “counter-flow” (or “contra-flow”) heat exchanger. The reason it is not a true counter-flow heat exchanger is that the incoming cold stream is split so that part of it starts half-way along, and part of it ends half-way along. To be a true counter-flow heat exchanger, all of the first flow has to travel in a substantially opposite direction to the second flow along the entire length of the heat exchanger for both flows, in order to provide a constant temperature difference along the length of the heat exchanger. For this, the input of the cold stream is generally at the opposite end of the heat exchanger from the input of the warm stream in a counter-flow heat exchanger.
In summary, although single-coil heat exchangers of the helical coil-on-tube type have reasonable production rates and perform well since they can be implemented as counter-flow heat exchangers, they can also incur significant pressure losses. Multiple coil-on-tube heat exchangers are able to overcome some of the pressure loss problems of single-coil designs, but they require additional headers to treat the liquids, and their performance is not as efficient as they could be, since they are not true counter-flow heat exchangers.
Therefore, it is desirable to have a type of heat exchanger that provides similar performance and production times to the single-coil design, while improving on the lower efficiency and need for additional equipment of the multiple-coil design.